1. Field of the Invention
The present invention relates to a system provided with a refrigeration circuit for circulating refrigerant in accordance with operation of a variable capacity type compressor driven by a prime mover, and more particularly to a method and apparatus suitable for calculating drive torque for the variable capacity type compressor. The method and apparatus may be adapted for use in an idling rotational speed control device for stabilizing the idling rotational speed of an engine of torque for a variable capacity type compressor of an air conditioning system for a vehicle, or a device for controlling an amount of a current flowing into an electromagnetic clutch which couples the engine to the compressor in accordance with the torque of the compressor.
2. Description of Prior Art
In such an idling rotational speed control device as disclosed in, for example, the Japanese Publication No. 41951/1987, there is proposed the device which controls an idling rotational speed of an engine in accordance with pressure of refrigerant in a high-pressure side in a refrigerant circuit.
With such construction, while torque for a compressor varies almost in proportion to the pressure of refrigerant in the high-pressure side of the compressor, it also varies in accordance with a change in a capacity of the compressor in case the compressor is of a variable capacity type. Although the conventional structure is designed in the light of pressure of refrigerant in the high-pressure side of the compressor, no consideration is given on a possible change in torque caused by a change in the capacity of the compressor. Therefore, torque for the compressor may not be accurately utilized for control of an idling rotational speed of an engine, thus lowering accuracy of the idling rotational speed. As a result, fuel consumption becomes worse when the idling rotational speed is maintained high, while engine stalling or undesired vibration would occur when the idling rotational speed is maintained low. In this respect, it is desired to calculate driving torque for the variable capacity type compressor taking account of a change in torque for the compressor caused by the change in the capacity of the compressor as well as the change in pressure of refrigerant in the high-pressure side of the compressor.